Sealable, biaxially oriented polyester films are known from the prior art. Likewise known are sealable, biaxially oriented polyester films which have one or more UV absorbers. These films known from the prior art either have good sealing performance, good optical properties, or acceptable processing performance.
GB-A 1 465 973 describes a coextruded polyester film having two layers, one layer of which consists of copolyesters containing isophthalic acid and terephthalic acid, and the other layer of which consists of polyethylene terephthalate. The patent gives no useful indication of the sealing performance of the film. Lack of pigmentation means that the film cannot be produced by a reliable process (cannot be wound up) and that the possibilities for further processing of the film are limited.
EP-A 0 035 835 describes a coextruded, sealable polyester film where, in the sealable layer, particles whose average size exceeds the sealable layer thickness are present in order to improve winding and processing performance. The particulate additives form surface protrusions which prevent undesired blocking and sticking of the film to rolls or guides. No further details are given concerning the incorporation of antiblocking agents in relation to the other, nonsealable layer of the film. It is uncertain whether this layer comprises antiblocking agents. The choice of particles having diameters greater than the sealable layer thickness, at the concentrations given in the Examples, impairs the sealing performance of the film. The patent does not give any indication of the sealing temperature range of the film. The seal seam strength is measured at 140° C. and is in the range from 63 to 120 N/m (from 0.97 N/15 mm to 1.8 N/15 mm of film width).
EP-A 0 432 886 describes a coextruded multilayer polyester film which has a first surface on which has been arranged a sealable layer, and has a second surface on which has been arranged an acrylate layer. The sealable outer layer here may also be composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters. The coating on the reverse side gives the film improved processing performance. The patent gives no indication of the sealing temperature range of the film. The seal seam strength is measured at 140° C. For a sealable layer thickness of 11 μm the seal seam strength given is 761.5 N/m (11.4 N/15 mm). A disadvantage of the reverse-side acrylate coating is that this side is no longer sealable with respect to the sealable outer layer, and the film therefore has only very restricted use.
EP-A 0 515 096 describes a coextruded, multilayer, sealable polyester film which comprises a further additive on the sealable layer. The additive may comprise inorganic particles, for example, and is preferably distributed in an aqueous layer onto the film during its production. Using this method, the film is claimed to retain its good sealing properties and to be easy to process. The reverse side comprises only very few particles, most of which pass into this layer via the recycled material. This patent again gives no indication of the sealing temperature range of the film. The seal seam strength is measured at 140° C. and is above 200 N/m (3 N/15 mm). For a sealable layer of 3 μm thickness the seal seam strength given is 275 N/m (4.125 N/15 mm).
WO 98/06575 describes a coextruded, multilayer polyester film which comprises a sealable outer layer and a nonsealable base layer. The base layer here may have been built up from one or more layers, and the inner layer of these layers is in contact with the sealable layer. The other (outward-facing) layer then forms the second nonsealable outer layer. Here, too, the sealable outer layer may be composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters, but these comprise no antiblocking particles. The film also comprises at least one UV absorber, which is added to the base layer in a weight ratio of from 0.1 to 10%. Preferred UV absorbers used here are triazines, e.g. âTinuvin 1577 from Ciba. The base layer has conventional antiblocking agents. The film has good sealability, but does not have the desired processing performance and also has unsatisfactory optical properties, such as gloss and haze.
DE-A 23 46 787 describes a flame-retardant polymer. Besides the polymer, the use of the polymer is also described for producing films and fibers. The following shortcomings are apparent when this phospholane-modified polymer is used in film production                The polymer mentioned is susceptible to hydrolysis and has to be very thoroughly predried. When the polymer is dried using dryers of the prior art it cakes, and production of a film is therefore impossible except under very difficult conditions.        The films thus produced under uneconomic conditions embrittle when exposed to heat, i.e. their mechanical properties are severely impaired by embrittlement, making the film unusable. This embrittlement occurs after as little as 48 hours of exposure to heat.        
It was an object of the present invention to provide a transparent, flame-retardant, sealable and biaxially oriented polyester film which does not have the disadvantages of the films mentioned as the prior art, and in particular has a combination of advantageous properties, such as very good sealability, a cost-effective production process, improved processability, and improved optical properties. In particular, it should be flame-retardant and not embrittle on exposure to heat.
It was an object of the present invention to extend the sealing temperature range of the film to low temperatures, to increase the seal seam strength of the film, and at the same time to provide better film handling than that known from the prior art. It should also be ensured that the processability of the film extends to high-speed processing machinery. It should be possible for regrind produced directly during film production to be introduced to the extrusion process at a concentration of up to 60% by weight, based on the total weight of the film, without any significant resultant adverse effect on the physical properties of the film.
Flame retardancy means that in what is known as a fire protection test the transparent film complies with the conditions of DIN 4102 Part 2 and in particular the conditions of DIN 4102 Part 1, and can be classified in Construction Materials Classes B2, and in particular B1 for low-flammability materials.
The film is also intended to pass the UL 94 test (Vertical Burning Test for Flammability of Plastic Material), permitting its classification into class 94 VTM-0. This means that 10 seconds after removal of the Bunsen burner the flame has ceased to burn, after 30 seconds no smoldering is observed, and no burning drops occur.
Cost-effective production includes the capability of the raw materials or raw material components needed to produce the flame-retardant film to be dried using industrial dryers of the prior art. It is important that the raw materials do not cake or become thermally degraded. These industrial dryers of the prior art include vacuum dryers, fluidized-bed dryers, and fixed-bed dryers (tower dryers).
These dryers operate at temperatures of from 100 to 170° C., at which the flame-retardant polymers mentioned in the prior art cake, making film production impossible.
In the vacuum dryer, which has the gentlest drying conditions, the raw material for the film passes through a temperature range of from about 30 to 130° C. under a vacuum of 50 mbar. What is known as post-drying is then required, in a hopper at temperatures of from 100 to 130° C. with a residence time of from 3 to 6 hours. Even here, the polymer mentioned cakes to an extreme extent.
No embrittlement on exposure to heat means that after 100 hours of a heat-conditioning procedure at 100° C. in a circulating-air oven the film has not embrittled and does not have disadvantageous mechanical properties. The good mechanical properties include high modulus of elasticity (EMD>3200 N/mm2; ETD>3500 N/mm2), and also good values for tensile stress at break (in MD>100 N/mm2; in TD>130 N/mm2).